I am currently reading a book about Zheng He, a famous Chinese admiral who’s voyages took him all the way to the East coast of Africa decades before Columbus would sail across the Atlantic. Zheng He’s treasure ships were massive, but seemed extremely unstable on the open ocean. They were flat-bottomed boats which could easily navigate up river; however, I imagine ocean travel was risky for these ships when they encountered large swells. European vessels during this time could not match the size of these ships, but also seemed much more stable in the ocean with their rounded, high hulls and deep keels. I suppose my question is: when did Europeans develop ships which we would recognize as seafaring vessels and did other civilizations (ancient China) know that a keel would make a ship much more stable in the open ocean?
So I am not an expert about Zheng He, but I will say that there's a lot of bad information out there about him -- the idea that he discovered the Americas, for example, is bunk.
But a keel on a ship and rounded sides don't necessarily make it "better" than a flat-bottomed ship, and vice versa -- ships are always built in a context not only of shipbuilding tradition, but also based on local factors. For example, Dutch ships were built with a shallower draft and more rounded hull than English, because they had to navigate much shallower water to get to ports in the Zuiderzee or through the IJsselmeer. That made them less weatherly than English ships, but it also meant that they could maneuver differently in the Channel -- in the action of Sept. 28, 1652 the English Sovereign and James both ran ashore on the Kentish Knock chasing the squadron led by the Dutch admiral Witte Corneliszoon de With.
But in any case, to answer your question about shipbuilding and keels, ships were being built in the Mediterranean context with keels in antiquity -- the classic Greek triremes used their (extended) keels as weapons, of course, but there were also merchant ships built with keels. Moving toward a period I'm more familiar with, Norse longships (at least the ones we know about -- we actually don't have a ton of surviving examples in northern Europe) were built to a fairly common or similar pattern of having a long keel and then planks sewn or nailed to that -- the ship was built up from the keel in what's called clinker construction. On the other hand, there were European ships built with a flat bottom, such as the cog, which was a cargo carrier that was meant to be able to beach itself in medieval harbors with limited port facilities. (The flat bottom also allowed for a hull shape that let it carry more cargo.)
I've written a bunch about this before, if it's of interest.
https://www.reddit.com/r/AskHistorians/comments/3pn7v5/what_changes_occurred_in_the_construction/
https://www.reddit.com/r/AskHistorians/comments/2709jm/how_would_a_britishhms_frigate_built_in_1715/
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